1. Field of the Invention
The present invention relates to a display device and a driving method thereof. In particular, the invention relates to a display device using a time gray scale method.
2. Description of the Related Art
In recent years, a so-called self-luminous display device is attracting attention, which has pixels each formed with a light-emitting element such as a light-emitting diode (LED). As a light-emitting element used in such a self-luminous display device, there is an organic light-emitting diode (also called an OLED (Organic Light-Emitting Diode), an organic EL element, an electroluminescence (EL) element, or the like), which has been attracting attention and used for an EL display or the like. Since the light-emitting element such as an OLED is a self-luminous type, it is advantageous as compared to a liquid crystal display in that high visibility of pixels is ensured, no backlight is required, high response speed is achieved, and the like. The luminance of a light-emitting element is controlled by the amount of current flowing therein.
As a method of controlling gray scales (luminance) in such a display device, there are a digital gray scale method and an analog gray scale method. In the digital gray scale method, gray scales are expressed by controlling on/off of a light-emitting element in a digital manner. On the other hand, as for the analog gray scale method, there are a method of controlling the light-emission intensity of a light-emitting element in an analog manner, and a method of controlling the light-emission time of a light-emitting element in an analog manner.
In the digital gray scale method, only two states of a light-emitting element can be selected, which are a lighting state and a non-lighting state; therefore, only two gray scales can be expressed. Thus, the digital gray scale method is used in combination with another method to achieve multi-gray scale display. As a method for achieving multi-gray scales, a time gray scale method is often used in combination.
As examples of a display where gray scales are expressed by digitally controlling a lighting state of pixels in combination with the time gray scale method, there are an EL display using a digital gray scale method, a plasma display, and the like.
The time gray scale method is a method of expressing gray scales by controlling the length of a lighting period or the number of lighting operations. That is, one frame is divided into a plurality of subframes, and each subframe is weighted in the number of lighting operations, the length of lighting periods, or the like, so that the total weight (the sum of the lighting operations or the sum of the lighting periods) is varied between different gray scales, thereby expressing gray scales. It is known that display defects called pseudo contours (or false contours) occur when using such a time gray scale method. Thus, countermeasures against such display defects have been examined (see Patent Document 1).
Pseudo contours can be reduced by increasing the frame frequency. As one of the methods, there is a method by which the length of a subframe is reduced to half so that the number of subframes in one frame can be doubled. This is substantially synonymous with doubling the frame frequency (see Patent Document 2). Such a method is called a double-speed frame method in this specification.
Here, considered is a case of displaying 5-bit gray scales (32 gray scales). First, FIG. 46 shows a selection method of subframes with a conventional time gray scale method, which specifically shows whether pixels are lighted or not in each subframe for expressing each gray scale. In FIG. 46, one frame is divided into five subframes (SF1 to SF5), which respectively have lighting periods with the following length: SF1=1, SF2=2, SF3=4, SF4=8, and SF5=16. That is, each lighting period has a squared length of a lighting period in the previous subframe. Note that a gray-scale level of 1 corresponds to a lighting period as long as 1. By combining these lighting periods, display with 32 gray scales (5-bit gray scales) can be performed.
Here, description is made on how to see FIG. 46. Pixels are lighted in subframes indicated by ∘ marks whereas pixels are not lighted in subframes indicated by x marks. By selecting subframes for lighting pixels for each gray scale, gray scales can be expressed. For example, in order to express a gray-scale level of 0, pixels are not lighted in SF1 to SF 5. In order to express a gray-scale level of 1, pixels are not lighted in SF2 to SF 5 whereas they are lighted in SF1. In order to express a gray-scale level of 7, pixels are not lighted in SF4 and SF5 whereas they are lighted in SF1 to SF3.
Next, FIG. 47 shows an example where a double-speed frame method is applied to the method in FIG. 46. By equally dividing each subframe in FIG. 46 into two, 10 subframes (SF1 to SF10) are obtained, which respectively have lighting periods with the following length: SF1=0.5, SF2=1, SF3=2, SF4=4, SF5=8, SF6=0.5, SF7=1, SF8=2, SF9=4, and SF10=8. Accordingly, the frame frequency is substantially doubled.
Furthermore, the same principle can be applied to the case of displaying 6-bit gray scales (64 gray scales). FIG. 49 shows an example where a double-speed frame method is applied to a subframe structure as shown in FIG. 48 where 6-bit gray scales are expressed by a time gray scale method. By equally dividing each subframe in FIG. 48 into two, 12 subframes (SF1 to SF12) are obtained, which respectively have lighting periods with the following length: SF1=0.5, SF2=1, SF3=2, SF4=4, SF5=8, SF6=16, SF7=0.5, SF8=1, SF9=2, SF10=4, SF11=8, and SF12=16. Note that a gray-scale level of 1 corresponds to a lighting period as long as 1. As in the case of displaying 5-bit gray scales, gray scales are expressed by selecting subframes for lighting pixels.
By equally dividing each subframe into two in this manner, the frame frequency can be substantially doubled.
In addition, as another method of increasing the frame frequency, there is a method disclosed in Patent Document 3.
Patent Document 3 discloses a case of displaying 8-bit gray scales (256 gray scales) as shown in FIGS. 1 and 4. FIGS. 50A and 50B illustrate a method of selecting subframes in this case. In order to display 8-bit gray scales with the conventional time gray scale method, one frame is divided into eight subframes, and a lighting period in each subframe is set to have a squared length of a lighting period in the previous subframe, such that 1, 2, 4, 8, 16, 32, 64, and 128. On the other hand, FIG. 4 according to Patent Document 3 shows an example where only four subframes (selected in decreasing order of lighting periods) are divided among the eight subframes. FIG. 50A shows a method of selecting subframes in this case.
FIG. 1 according to Patent Document 3 shows an example where 256 gray scales are expressed not by setting a lighting period in each subframe to have a squared length of a lighting period in the previous subframe, but by using such an arithmetical sequence that a difference between adjacent bits is 16 among 5 high-order bits such that 1, 2, 4, 8, 16, 32, 48, 64, and 80. Thus, only five subframes (selected in decreasing order of lighting periods) are divided. FIG. 50B shows a method of selecting subframes in this case.
By using such methods, the frame frequency can be substantially increased.    [Patent Document 1] Japanese Patent No. 2903984    [Patent Document 2] Japanese Patent Laid-Open No. 2004-151162    [Patent Document 3] Japanese Patent Laid-Open No. 2001-42818
However, even in using the double-speed frame method, pseudo contours are caused depending on which of the subframes are selected for lighting pixels (i.e., if selected subframes are very different between adjacent gray scales).
First, considered is a case of displaying 5-bit gray scales. For example, with the subframes shown in FIG. 47, a gray-scale level of 15 is expressed in a pixel A while a gray-scale level of 16 is expressed in the adjacent pixel B. FIGS. 51A and 51B show the lighting/non-lighting states of the pixels in each subframe in this case. FIG. 51A shows a case where only the pixel A or only the pixel B is seen by a human with his/her eyes being fixed. In this case, pseudo contours do not occur because human eyes can perceive brightness by the total amount of brightness that his/her visual axis catches. Thus, human eyes perceive that the gray-scale level is 15 (=4+2+1+0.5+4+2+1+0.5) in the pixel A, while the gray-scale level is 16 (=8+) in the pixel B. That is, an accurate gray-scale level can be perceived by human eyes.
On the other hand, FIG. 51B shows a case where the visual axis moves from the pixel A to the pixel B or from the pixel B to the pixel A. In this case, depending on the movement of the visual axis, the human eyes perceive that the gray-scale level is 15.5 (=4+2+1+0.5+8) or 23.5 (=8+8+4+2+1+0.5) sometimes. Although it is originally supposed that the gray-scale levels are perceived as 15 and 16, the gray-scale levels are actually perceived as 15.5 or 23.5, thereby pseudo contours occur.
Next, FIG. 52 shows an example of displaying 6-bit gray scales (64 gray scales). For example, assuming that a gray-scale level of 31 is expressed in a pixel A while a gray-scale level of 32 is expressed in the adjacent pixel B, human eyes perceive that the gray-scale level is 31.5 (=8+4++21+0.5+16) or 47.5 (=16+16+8+4+2+1+0.5) sometimes, depending on the movement of the visual axis as in the case of a 5-bit gray scale display. Although it is originally supposed that the gray-scale levels are perceived as 31 and 32, the gray-scale levels are actually perceived as 31.5 or 47.5, thereby pseudo contours occur.
Furthermore, FIG. 53A shows the case of FIG. 50A, and FIG. 53B shows the case of FIG. 50B. For example, assuming that a gray-scale level of 127 is expressed in a pixel A, while a gray-scale level of 128 is expressed in the adjacent pixel B, the gray-scale levels are perceived as being different from what they are supposed to be, depending on the movement of the visual axis, similarly to the examples described heretofore. For example, in the case of FIG. 53A, human eyes perceive that the gray-scale level is 121 (=64+32+16+8+1) or 134 (=32+16+8+8+4+2+64) sometimes. In the case of FIG. 53B, human eyes perceive that the gray-scale level is 120 (=40+24+32+16+8) or 134 (=32+16+8+8+4+2+40+24) sometimes. In either case, although it is originally supposed that the gray-scale levels are perceived as 127 and 128, the gray-scale levels are actually perceived as being different from what they are supposed to be, thereby pseudo contours occur.
In addition, when using the double-speed frame method, the number of subframes is increased; therefore, the duty ratio (ratio of lighting periods to one frame) is decreased accordingly. Thus, in order to keep the same average luminance as in the case of not using the double-speed frame method, a voltage applied to a light emitting-element is required to be increased, which results in the increased power consumption, lower reliability of the light-emitting element, and the like.